Hitherto, the preparation of printing plates for lithographic printing has been carried out in a system in which a printing plate precursor is exposed to light through a lith film as an intermediate material. However, in recent years, as the digitalization rapidly proceeds in the printing field, the preparation step of the printing plate is being changed to a computer-to-plate (CTP) system in which digital data input and edited in a computer are directly output to a printing plate precursor. As the printing plate precursor for CTP, are proposed various modes as described, for example, in Journal of Printing Science and Technology, Vol. 36, pp. 97-104 and pp. 148-163 (1999). Of these are also actively studied lithographic printing plate precursors capable of drawing images by heat or heat mode exposure.
Even in such a lithographic printing plate precursor capable of drawing images by heat or heat mode exposure, like in conventional lithographic printing plate precursors, it is important in the printing plate preparation work to discriminate whether or not the precursor is already exposed to light, to inspect images on the printing plate, and to distinguish the plate to which what color of inks should be applied. In particular, since development is not carried out in a printing plate requiring no development, inspection and discrimination of the plate are difficult in mounting the plate on a printing press, and hence, a method for compensating these difficulties is required.
To solve such difficulties, JP-A-11-2717927 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) describes a printing plate comprising, on a support, a recording layer containing a light-heat converting agent, a compound generating an acid, a base or a radical by light or heat, and a compound that discolors by a mutual action with the generated acid, base or radical, which can be exposed with an infrared laser and has exposed image-visualizing property (printout property).
However, the compound generating an acid or a radical, which is suitable for obtaining a printout image as described in the above patent, has an absorption in a visible light region. Accordingly, in the printing plate using such a compound, the acid or radical is generated during standing in a light room before or after the imagewise exposure, to cause discoloration. As a result, there are involved problems such that the generation of the printout image during imagewise exposure is inhibited and that the generated printout image becomes unclear during a time when the plate exposed imagewise is allowed to stand.
JP-A-2001 -33953 describes that in a light-sensitive material comprising, on a support, a recording layer containing an electron-accepting polymer soluble in an alkaline developing solution and a near infrared-absorptive dye, when near infrared light is irradiated, the optical reflection density in the irradiated area of the recording layer decreases so that drawn images can be visually confirmed at the time of completion of the exposure. However, the dye used in the technology is an electron-donating dye precursor, from which a readily visual printout image can be obtained only in the presence of the electron-accepting polymer, and hence, such a dye is not a general-purpose dye for printout.